The present invention relates to a method for producing a semiconductor device and, more particularly, to a method for producing an improved semiconductor device which has a means for forming a material film pattern.
Formation of the material film pattern in a conventional method for producing a semiconductor integrated circuit is accomplished wherein a resist film is coated on a film to be etched, the resist film is selectively exposed to light and developed to obtain a predetermined resist pattern, and the film to be etched is etched using the obtained resist pattern as a mask. In particular, dry etching techniques have been developed extensively, for example, reactive ion etching utilizing an etching system of parallel planar electrode type which substantially eliminates side etching. In this manner, the resist pattern is precisely transferred to the film to be etched.
However, when the etching techniques are changed, the resistance of the resist film to etching entails problems. When reactive ion etching is used, the resist film is also etched. Therefore, a desired material film pattern is not obtained.
In order to solve this problem, a second material film which allows sufficiently selective etching is formed on a first material film. The second material film is patterned by photolithography to form a second material film pattern. The first material film is then etched utilizing the second material film pattern as a mask. In this way, a mask-transfer technique is achieved. However, this technique has problems in that the steps are very complicated, stress is generated or chemical reaction may occur between the first material film and the second material film so that the film quality is degrated, and the second material film on the first material film pattern is hard to remove due to the etching selectivity between the exposed material under the first material film pattern and the second material. When reactive ion etching is used for removing the first material film using the second material film pattern as the mask, part of the second material film is decomposed during etching and a new etching species is produced so that side etching of the first material film adversely occurs due to this etching species.
In the conventional MOS type semiconductor integrated circuit, on an area between adjacent elements, that is, in the field region, a relatively thick field insulating film is formed for the purpose of due to element isolation. Further, a channel stopper of high impurity concentration which has the same conductivity as the semiconductor substrate is formed under the field insulating film to increase the inversion voltage of the field region. As a method for forming the field insulating film and the channel stopper thereunder, the selective oxidation method has recently been utilized. According to this method, a silicon nitride film is deposited on a silicon substrate with a thin silicon oxide film interposed therebetween. The silicon nitride film is selectively etched except for an element formation region and the remaining area of the silicon nitride film is used as a mask for ion implantation. Ion implantation is performed for forming the channel stopper and, by using the same silicon nitride film as the mask, a thick oxide film is selectively deposited on the field region by high temperature thermal oxidation. Thereafter, the silicon nitride film is removed and the thin silicon oxide film thereunder is then removed to form elements on the exposed substrate by a known process. According to this method, the photoetching process is required only once when the silicon nitride film is selectively etched. Further, since the insulating film of the field region and the channel stopper are self-aligned, the method described above has advantages in that the manufacturing cost is low and the masking error need not be considered.
However, the following problems are present when the selective oxidation method is used for isolating elements in an integrated circuit in which compactness and high packaging density are developed.
Firstly, a nitride compound produced from the silicon nitride film used as the oxidation resistant mask in the selective thermal oxidation described above is diffused in the silicon oxide film under the silicon nitride film and a nitride compound is produced on the surface of the silicon substrate. This nitride compound prevents formation of a gate oxide film, degrades the dielectric strength of the gate oxide film, and causes fluctuations in the gate threshold voltage, when part of the silicon substrate for the element formation region is exposed and the gate oxide film is formed by thermal oxidation. Secondly, when a thick field oxide film is selectively formed, oxidation proceeds in the transverse direction and the thick field oxide film extends in the form of a bird beak under the silicon nitride film as an oxidation resistant mask. Therefore, dimensional errors of the element formation region occur, preventing high packaging density. Thirdly, in order to form the thick field oxide film, annealing is required at high temperature and for a long period of time, for example, 1,000.degree. C. for 5 hours, in an oxidizing atmosphere containing water vapor. Therefore, an impurity which is already ion-implanted in the field region will be diffused into the element formation region, the field stopper will be rearranged, and the element characteristics will be degraded.